This invention relates to a glass blank disc centering device for an optical disc glass blank disc exposure apparatus that centers a glass blank disc to be made into an optical disc master disc so that the center of the glass blank disc coincides with the rotational center of a turntable.
Conventionally, in an optical disc glass blank disc exposure apparatus, when the center of a glass blank disc to be made into an optical disc master disc is not aligned with the rotational center of the turntable, dynamic unbalance causes vibration during rotary recording on this glass bank disc. As a result, the track pitch and recording precision of the track recorded on the glass blank disc decreases.
There is a method of avoiding this problem wherein a center hole is provided passing through the center of the glass blank disc and the glass blank disc is centered by fitting a center pin provided on the turntable into this center hole. However, with this method, in order to achieve high-precision centering, it is necessary that the concentricity of the center hole and the outer cylinder of the glass blank disc and the dimensional precision of the center hole be high. Consequently, there has been the problem that the glass blank disc is expensive.
Also, when a center hole is provided in the glass blank disc, in uniformly spin-coating photoresist on the glass blank disc, in carrying out wet processing such as developing after exposure, and in regrinding the glass blank disc during recycling thereof, the center hole tends to cause problems such as deterioration in surface precision due to chemical residue and surface discontinuity; and therefore it is preferable that there be no center hole.
Furthermore, when the above-mentioned center hole is provided, if there are defects such as fine cracks in the center hole, centrifugal force during rotation of the glass blank disc may cause these to develop into substantial cracks and there is a danger that in some cases the glass blank disc will break up and fly apart.
Thus, there are various problems associated with centering using a center hole provided in the glass blank disc, and consequently, in practice, this method is not used much.
Centering of the glass blank disc can also be carried out using the outer cylinder of the glass blank disc, by the glass blank disc being retained on the turntable by the outer cylinder of the glass blank disc being retained all around its circumference by means of a ring or in at least three locations by means of guide rollers.
However, with this method, because it necessitates a rotating part larger than the glass blank disc, there has been the problem that compared to when a center hole is used, the centering mechanism is much larger and the cost of machining its constituent parts to a high precision is higher. Also, when an angular ring is provided around the periphery of the turntable, there has been the problem that fitting and removal of the glass blank disc are troublesome and there is a greater risk of scratching or otherwise damaging the recording surface of the glass blank disc.
When the above-mentioned guide rollers are provided or when a ring with a cutaway portion is provided so that fingers or claws of a jig can more easily reach the recording rear surface of the glass blank disc, because the balance of the turntable itself is consequently impaired, there has been the problem that correction of the balance of the turntable itself is necessary. Moreover, because its shape becomes discontinuous it greatly disturbs the surrounding airflow during high speed rotation, noise and windage loss increase. Consequently the accuracy of the speed of rotation falls.
Recently, optical discs and the like of higher density and higher speed accessibility have been being produced, and high speed rotation is becoming required also during recording on glass blank discs. Here, the acceleration of the vibration occurring due to dynamic unbalance of the glass blank disc is proportional to the square of the rotational speed. Consequently, with the same centering accuracy as in the past, the vibrational acceleration increases in proportion with the square of the conventional acceleration.
As a result the influence of vibrational acceleration becomes extremely large, and recording accuracy decreases markedly. Also, there have been problems such as that because the vibration frequency also increases in proportion to the rotational speed, when the inherent vibration frequencies of the various parts of the glass blank disc exposure apparatus and the frequency at which the glass blank disc is vibrating coincide, resonance causes the vibration to increase.
Because countermeasures such as making the various parts of the exposure apparatus more rigid than in the past consequently become necessary, there have been problems such as that the cost of the exposure apparatus itself increases.
Another method of suppressing vibration caused by resonance is to suppress the displacement due to vibrational acceleration caused by dynamic unbalance of the glass blank disc to a level such that it does not contribute to deterioration of the track pitch and recording precision by making the base of the apparatus, on which the turntable is mounted, amply heavy. However, with this approach, there are problems such as that increasing the weight of the apparatus tends to place limitations on where it can be installed and makes it difficult to make the apparatus compact. Moreover, material costs also increase.
Consequently, an operation of so centering the glass blank disc from outside that the center of the outer cylinder of the glass blank disc coincides as closely as possible with the rotational center of the turntable is necessary, and the methods described below are employed for this.
A first method is one in which an operator placing a glass blank disc on the turntable sets a displacement gauge such as a dial gauge or an electronic micrometer or the like against the outer cylinder edge surface of the glass blank disc and while rotating the glass blank disc on the turntable so finely adjusts the position of the glass blank disc that its eccentricity is reduced to a minimum or to within a tolerable range.
A second method is one in which when the glass blank disc is placed on the turntable the eccentricity of the glass blank disc is reduced to within a fixed allowable range using a jig or the like, the turntable is rotated at a predetermined speed, the unbalance of the glass blank disc is detected using a balance tester or a dynamic balancer or the like which detects the size and direction of the vibration caused by the dynamic unbalance, and balancing weights are attached or the position of the glass blank disc is adjusted.
However, in both these two methods, the work involved takes time and a skilled specialist operator is required. Also, because a person is present, there have been problems such as that dust from the person alights on the recording surface of the glass blank disc and defects arise in the recorded signal recorded on the glass blank disc.
Also, in the second method there has been the problem that generally, because detection of acceleration generally is of poorer resolution than detection of displacement, to obtain the same resolution as obtained with displacement detection, an expensive detecting instrument is necessary.
To overcome these problems, the methods described below have been disclosed.
First, there are methods such as those disclosed in Japanese Laid-Open Patent Publications No. S.61-206956, No. H.1-210240 and No. H.1-217760 wherein the above-mentioned first method is automated and the presence of the person is dispensed with.
However, in all these methods, the eccentricity is corrected and checked by the turntable being rotated through at least two revolutions at a relatively low speed. Consequently, there have been problems such as that the time required for centering is relatively long and that because a displacement measuring part, a driving part and a controlling part are necessary, to perform high-precision centering complex and expensive equipment is required.
There is another method, disclosed in Japanese Laid-Open Patent Publication No. H.4-387640, wherein the glass blank disc is moved into position from at least three directions simultaneously or sequentially without the turntable being rotated.
However, with this method, as well as the problem remaining that a displacement measuring part, s driving part and a controlling part are necessary and to perform high-precision centering complex and expensive equipment is required, there is also the problem that because the glass blank disc is moved into position according to calculations, the centering operation takes time.
On the other hand, in Japanese Laid-Open Patent Publication No. H.5-28541 and elsewhere, a method has been proposed wherein a mechanism of a type typified by a scroll-type three-clawed chuck used to fit a workpiece to a lathe major axis table of a machine tool or to fit a drill bit to a boring plate is applied. In this method, centering is completed in an instant without the turntable being rotated by the position of the glass blank disc being simultaneously adjusted from three directions. Also, because no displacement measuring part is necessary, the constitution of the centering apparatus can be made simpler than in the methods mentioned above.
However, in this kind of method, to carry out high-precision centering, there is a one-to-one correspondence relationship between the precision of the various members and the eccentricity; that is, high concentricity of the rotational center of adjusting members and a driving rotary table and guide rollers is required.
Furthermore, similarly high precision is required for guide grooves in three or more adjusting members which are provided. Consequently, there has been the problem that increasing the precision of the apparatus overall has meant increasing its cost.